Heterogeneous monopropellant compositions

ABSTRACT

1. Novel thixotropic, monopropellant compositions comprising a mixture of a solid fuel selected from the group consisting of inorganic silicides, silicon carbide and mixtures thereof, a strong liquid oxidizer and a thixotroping agent selected from the group consisting of particulated carbon and silica.

United States Patent Tannenbaum 5] Dec. 9, 1975 1 HETEROGENEOUS MONOPROPELLANT 3.092.959 6/1963 SCUl'lOCk et a1. 149/36 x CO P 3,097,479 7/1963 Reipold 3,116,187 12/1963 Scanlon et a1. 149/36 X Inventor Stanley Tannenbaum, Mo 3.136669 6/1964 Carpenter 149/74 NJ 3.197348 7/1965 Skolnik et 31...... 149/52 X Assignee: o ol o poratmn, Bristol, Pa $691,769 9/1972 Kellbach et a1 60/217 [22] Filed: Apr. 22, 1964 Primary ExaminerBenjamin R. Padgett Assistant Examiner- 15. A. Miller [21] Appl' 363333 Attorney, Agent, or Firm-Stanley A. Marcus; William R. Wright, Jr. [52] US. Cl. 149/74; 149/89; 149/1082;

149/1088; 149/1092; 60/217 EXENIPLARY CLAIM [51] Int. Cl. C06D 5/10 [58} Field 0 Search h 60/3514 149/74 87 1. Novel th1xotrop1c, monopropellant composltlons 149/17 18 44 89 6 2 1 8 comprising a mixture of a solid fuel selected from the 7 group consisting of inorganic silicides, silicon carbide [56] References Cited and mixtures thereof, a strong liquid oxidizer and a thixotroping agent selected from the group consisting UNITED STATES PATENTS of particulated carbon and silica. 2,938,779 5/1960 Kolfenback et a1 149/74 X 3,035,943 5/1962 FOX 1. 149/87 x 6 Claims, N0 Drawmgs HETEROGENEOUS MONOPROPELLANT COMPOSITIONS This invention relates to novel heterogeneous monopropellant compositions.

More particularly, this invention concerns the preparation of highly energetic, thixotropic propellant compositions which are superior in certain respects to presently utilized solid or liquid propellant compositions.

The novel, thixotropic propellant compositions of this invention are composed essentially of:

A. Fuels selected from the group consisting of inorganic hydrides, inorganic silicides, hydrocarbons, silicon carbide and mixtures of these hydrides and silicides.

B. Liquid oxidizer.

C. Thixotroping or gelling agents and optional propellant adjuvants.

The use of liquid propellant compositions offers several significant advantages over comparable solid propellant compositions. For example, liquid propellant formulations are much more energetic than currently used solid compositions and have greater specific impulse. Increased specific impulse gives the missile a longer range and a higher velocity for the same weight of propellant charge.

A further superiority of liquid propellants over solid propellants is that the combustion of liquid propellants can be mechanically controlled during flight. Combustion can be stopped and started at will, by controlling the flow of the propellant into the combustion chamber. Reducing the flow of the propellant into the combustion chamber decreases the thrust of the propellant while, increasing the flow rate has the opposite effect. Since the heterogeneous propellants of this invention are in the liquid state when utilized, their flow rate can be controlled mechanically. This control is accomplished by adjusting the pumps and valves in the missiles fuels transport system.

The significance of mechanical control is that all liquid propellant systems have a built-in" throttleability feature that is absent in solid propellants. Throttleability allows the missiles velocity to be varied, controls the attitude of the missile, makes evasive action possible, permits the rendezvous of two or more space ships during flight and reduces the hazards of landing.

in contrast, since solid propellants cannot flow, no comparable fuel transport takes place and no comparable control of combustion through varying the flow rate is possible. In fact, using the present technology, no method is presently available to control the combustion of the solid propellant charge after ignition. Thus, the difierence of physical state alone makes all solid propellants inherently disadvantageous to liiquid propellants for many applications.

Further significant disadvantages of solid propellant compared to liquid propellants arise in a number of ways because of the extreme sensitivity of solid propellants toward temperature and pressure fluctuations. This occurs both during storage and use. For example, the temperature of a solid propellant grain substantially influences performance. A given grain of solid propellant will produce more thrust on a hot day than on a cold day. Further. the physical state of the solid propellant is affected by temperature extremes. For instance, as a very low temperature many solid propellants become brittle and subject to cracking. Cracks in the propellant grain increase the propellants burning rate significantly and can cause a fracture or explosion. On the other hand a solid propellant exposed to high temperatures prior to firing can lose its shape and have its performance adversely affected. This sensitivity of solid propellants toward temperature fluctuation necessitates expensive storage under constant temperature prior to use.

For all of the foregoing reasons solid propellants are presently inferior to comparable liquid propellant compositions for many applications.

Even more advantageous than liquid propellants bypropellant se are liquid monopropellant compositions. These monopropellant formulations unlike bi-propelllant formulations contain oxidizer, fuel and any other required adjuvant materials, combined and stored as a single formulation. Since these formulations already have sufficient oxygen no additional source of oxidizer is required and they can be handled after formulation as a single complete composition. Thus, only one storage tank and but one pumping system is needed both on the ground and in the missile. in contrast, bipropellant formulations consist of at least two separate compositions since the fuel and oxidizer components are kept physically separate until they are injected into the missiles combustion chamber. Both at the storage facility and in the missile, duplicate storage and pumping systems are essential. Furthermore, in many instances wherein the oxidizer is a gas such as oxygen, the oxidizer must be refrigerated under high pressure. Halving the pumping and storage requirements, particularly the pumping system, reduces the mechanical complexity of the fuel transport system and decreases the likelihood of mechanical malfunction of the missile. In addition, the need for only one set of storage and pumping facilities greatly simplifies design and construction of the missiles, decreases the weight of the hardware and increases the fuel load of the missile. An ancillary but not unimportant result of simplifying the transport and storage systems is the reduction of maintenance time and maintenance and storage costs.

Unfortunately, while monopropellant formulations offer all of the above enumerated advantages over bipropellant formulations,.these advantages heretofore have been largely unrealized. The reasons for this have been several. Among other things, monopropellant compositions have been too easily ignited, become unstable upon prolonged storage, too readily detonated upon being disturbed and give erratic and relatively poor performance.

For example, the prior art monopropellant compositions are typified by the following: heptane-nitrogen tetraoxide, n-propyl nitrate, nitromethane, propargyl nitrate-nitramine and the like. All of these materials suffer from the failing of poor thermal stability, high sensitivity toward detonation and/or the tendency to deteriorate under prolonged storage. Poor thermal stability requires refrigerated storage, while a high sensitivity toward detonation by shock makes the propellants hazardous to store, transport and use. The deterioration of propellants after prolonged storage causes erratic performance and makes it continually necessary to substitute fresh propellant for aged propellant in order to maintain the initial high specific impulse. These stability factors among others greatly negate the value of the liquid monopropellants particularly in military retaliatory weapons.

An additional disadvantage of bipropellant liquids compared to monopropellants is in the cricticality of the oxidizer to fuel ratio and the narrow margin of malfunction allowed in the performance of the fuel metering and injection system.

While the oxidizer to fuel ratio is extremely critical to performance in all liquid propellants, in monopropellants and demands are much less stringent. This is because the oxidizer is added to the fuel during formulation and prior to use. Therefore the critical oxidizer to fuel ratio in monopropeilants can be accurately determined and corrected if necessary to assure optimum performance prior to firing. ln bipropellants this adjustment of oxidizer to fuel ratio cannot be made prior to firing. The reasons for this is that the oxidizer and fuel are separately stored until they are injected into the missile combustion chamber for use. Thus the ratio of oxidizer to fuel in the final propellant mixture is determined only at the instant of firing and cannot be corrected. Since the metering device like any complex mechanism is subject to failure. a deviation or even abortion of the missile flight can result.

Since this malfunction cannot be foreseen unitl it occurs no preventive measures are possible. In a similar vein, because of the losses of fuel and oxidizer which are known to occur because of the injection and combining of the separate streams of fuel and oxidizer in the missile combustion chamber, it is necessary to store an additional supply of both propellant components in the rocket.

These losses of fuel and oxidizer are an inherent part of bipropellant systems and are referred to as outage losses. The extra weight of the outage resevoir reduces the payload which the missile could carry and hence is disadvantageous. All of these shortcomings of bipropellant liquids are absent in monopropellants since the propellant is premixed and requires no metering.

For the above reasons among many others, the preparation of liquid monopropellant formulations having high specific impulse is to be desired. Especially valu able would be liquid propellant formulations which re tain their initial high specific impulse and/or density impulse yet remain relatively insensitive to detonation by shock. This type of a highly energetic liquid propellant would be a major advance in the propellant art. ideally these formulations would combine a low freezing point with the aforementioned properties and could be prepared from commercially available innocous components and would have thixotropic properties. The low freezing point would prevent freeze-up during flight or storage while the thixotropic state would allow the formulations to be stored as a solid and pumped as a liquid.

Thus it is an object of this invention among others to prepare highly energetic, monopropellant formulations.

It is an additional object of this invention to prepare monopropellant compositions which retain their original high specific impulse and/or density impulse for relatively long periods of time.

Yet another object of this invention is to prepare thixotropic liquid monopropellant compositions which can be cast and stored as solid propellants yet can be pumped as liquid propellants.

Yet another object of this invention is to prepare liquid propellant compositions having a low freezing point and thixotropic properties.

It is still another object of this invention to prepare highly energetic liquid propellant compositions from readily available and individually safe components.

Other objects of this invention will become apparent to those skilled in the propellant art by a further reading of this patent application.

These objects among others are achieved by the heterogeneous monopropellant compositions and processes described herein.

In practice, novel and superior thixotropic propellant compositions are derived by preparing uniform mixtures consisting of (a) inorganic hydrides, inorganic silicides, carbon hydrides, carbon silicides and mixtures of these hydrides and silicides, said fuels being in finely divided form (b) liquid oxidizer and (c) thixotroping agents with or without propellant adjuvants.

More specifically, propellant compositions of this invention consist essentially of:

A. from about [5 to 50 parts by weight of a finely divided solid fuel selected from the group consisting of inorganic hydrides, inorganic silicides, carbon hydrides, carbon silicides and mixtures of these hydrides and silicides.

B. from about 50 95 parts by weight of liquid oxidizer.

C. up to about 10 parts by weight of thixotroping agents with or without optional propellant adjuvants.

These latter propellant adjuvants include surface active agents, conditioning agents, modifiers and the like which while not necessary for operable propellant compositions, are desirable for optimum performance. The propellant adjuvants change, modify or impart to the propellant certain desirable physical and combustion characteristics so that they can be most effectively good. Typical adjuvants include surface active agents, viscosity modifiers, combustion catalysts, stabilizing agents and the like. Where such adjuvants are used they will more customarily comprise between about /& to 6 parts by weight of the final propellant compositions.

The above components of the propellant composition are thoroughly mixed or blended to form a uniform thixotropic mixture then pumped into the rocket motor as a viscous liquid which soon sets to a gel. When the propellant is to be ignited it is exposed to a shearing force converting it to a pumpable liquid. The liquid is then pumped into the combustion chamber for use. The pumping procedures are well known in the propellant art. Since the propellant compositions contain at least three classes of ingredients, it is essential for satisfactory performance that the composition be uniform in content. Thus throughout this disclosure and claims the proepllant composition referred to are understood to be those uniform in content.

A. Fuel The fuels referred to throughout this application are selected from the group consisting of inorganic hydrides, inorganic silicides, hydrocarbons, silicon carbide and mixtures of these hydrides and silicides. lllustrative hydrides include among others: aluminum, lithium, boron, beryllium and mixtures of these hydrides. While all of the above fuels are better than average in the inventive compositions, as in any large group, certain members of the group are advantageous to the group as a whole and are preferred. A favored group of fuels in the inventive propellant compositions are the hydrides of covalent structure as well as mixtures of these fuels. However. even within this more narrow group, some fuels are by far superior to the favored fuels. These latter preferred fuels which are the preferred fuel embodiments of this invention are beryllium hydride and aluminum hydride, hydrocarbons and their mixtures.

B. Liquid oxidizers The liquid oxidizers of this invention are of diverse structure and origin. Among the various oxidizers which can be used are the following: nitric acid, nitric acid enriched with N0 (RFNA), and nitric acid enriched with N0 and HF (IRFNA and HlRFNA) nitrogen tetraoxide (N 0 concentrated hydrogen peroxide (H 0 perchloryl fluoride (ClO F), tetrafluorohydrazine (N F tetranitromethane (C(NO the various nitroglycols, concentrated perchloric acid dihydrate and the like. The preferred oxidizers of this invention are concentrated H 0 N 0 C and, in one instance, water for the hydride fuelds. Peculiarly enough water is an effective oxidizer when used in conjunction with beryllium hydride fuel. For the silicide fuels the preferred oxidizers are H 0 N 0 l-lClO particularly the dihydrate, and nitric acid enriched with N0 and HF (lRFNA and HlRFNA). Generally these oxidizers are preferred because of their lost cost, commercial availability and most important, because of the highly energetic propellant compositions that are produced when they are used in conunction with the aforenamed fuel components.

C. Thixotropic Agents or Gelling Agents These agents which are alternatively referred to as thickening agents are used to thicken the propellant compositions so that they can be stored as thixotropic solids, yet under a shearing force will revert to the liq uid state. These gelling or thickening agents can be present in amounts ranging from 0 to 6 parts by weight or higher. More generally the thickener will be used in amounts ranging from l to 4 parts by weight. The exact amounts used will depend upon the type and amount of the particular fuel and oxidizer used as well as the thickner employed. An abbreviated but illustrative list of thickners includes among others the preferred thickner, powdered carbon, the various anhydrous and particulate colloidal silicas, colloidal clays such as bentonite, the alkoxy celluloses such as methoxy-, ethoxyand propoxy celluloses, the vegetable gums, algenic acid and its salts, polyols, resins and the like.

D. Optional Propellant Adjuvant" This is the generic designation used to describe the various conditioning, modifying agents, solvents and the like used to produce optimum performance from the propellant compositions of this invention. These adjuvants ordinarily make up a minor proportion of the final propellant composition, seldom exceeding 10 parts by weight of the propellant composition, seldom exceeding 10 parts by weight of the propellant composition and more typically comprising 0 6 parts by weight of the compounded propellant exclusive of thixotropic agents. The adjuvants listed previously are the most important utilized although many other adjuvants can be employed if desired.

E. Compounding the Propellant Formulations In preparing the novel liquid propellant formulations of this invention, several compounding procedures amoung many can be followed. The following represents the preferred formulative procedure:

The weighted, dry solid fuel ingredient(s) of the formulation are mixed until a homogeneous and uniform solid mixture is obtained. The mixing or blending operation can be accomplished using any number of commercial tumblers, blenders, agitators or mixers. Since the components are not detonable both individually and as a mixture, no special precautions in mixing need to be taken. When the solid ingredients have been satisfactorily blended they are added to the required amount of oxidizer until a highly viscous liquid thixotropic mixture is obtained. This gelled propellant mixture is pumped into a rocket engine as a viscous liquid and rapidly sets to a heavy gel. When the gel is exposed to a shear force, it liquifies and its viscosity and flow rate into the missile combustion chamber can be con trolled mechanically as is the case in a typical liquid propellant composition.

Because of their exceptional stability, the novel het erogeneous propellants of this invention are preferably ignited using any one of several possible techniques. One method is referred to as the hypergolic technique. In this method a small amount of chemical agent reactive with one or more of the propellant components is injected into the missiles combustion chamber with the flow of propellant mixture. The ignition is initiated by the reaction of chemical agent with the propellant components and the propellant once ignited burns smoothly. A satisfactory chemical agent for this pur pose among others is unsymmetrical dimethyl hydrazine.

In a second method, the combustion is initiated using a squib of solid propellant. The ignition of the solid propellant can be electrically actuated.

F. Preferred Heterogeneous Monopropellant Compositions As indicated supra many different factors are involved in determining whether a given propellant composition is to be favored over another. Among these factors are high specific impulse, high density impulse, insensitivity toward detonation, cost, availability of the components as well as the type of use contemplated. For use as rocket propellants the most preferred heterogeneous monopropellants consist essentially of the following:

l5 to 50 parts by weight of a solid fuel selected from the group consisting of aluminum and beryllium hydrides, mixtures of these hydrides, and hydrocarbons.

50 to 95 parts by weight of an oxidizer selected from the group consisting of perchloryl fluoride, nitrogen tetraoxide, and concentrated hydrogen peroxide (preferably above and water, and from about I to 7 parts by weight of thixotroping or gelling agent(s).

The workings of this invention can be shown more clearly by the typical embodiments which follow below:

In one embodiment of this invention a heterogeneous propellant composition is prepared utilizing beryllium hydride as fuel and water as oxidizer. The preparation is as follows:

A 36 parts by weight portion of beryllium hydride fuel and 5 parts by weight of silica (having a particle size of 7 l0 millimicron and a surface area of 300 350 sq. meters/gram) are blended in a PREMIER Dispersator fitted with a 1 inch Duplex Head. The mixing time is 15 minutes. The blended bimodal mixture is added to a 59 parts by weight of water in the same type of dispersator. Again the blending is continued for l5 minutes. A highly viscous gel is obtained which has a specific impulse of about 327.

In another embodiment using the same equipment and blending time and techniques as before, an aluminum hydride based propellant is prepared by blending a previously blended mixture of 7 parts by weight of finely divided silicon thickener (7-10 millimicrons and a surface area of 300-500 sq. meters/gram and 60 parts by weight of powdered aluminum hydride with [38 parts by weight of N04 oxidizer. Again a viscous highly energetic gel is obtained. The propellant has a specific impulse of about 296 after blending.

In still another embodiment of this invention another propellant formulation is prepared as above: 36 parts by weight portion of powdered Bel-l fuel, 5 parts by weight of silica thickener described above and 59 parts by weight of 98% H oxidizer. The blended propellant formulation is a thick gel having a specific impulse of about 360.

Other embodiments are as follows: a I22 parts by weight portion of decarborane, boron hydride, is blended for 25 minutes with l4 parts by weight of finely divided silica thickener described previously and 98% H20: (255 parts by weight). The resultant propellant has a specific impulse of about 310.

A 122 parts by weight portion of decaborane (a boron hydride), 22 parts by weight of Bel-i and 14 parts by weight of the afore-mentioned fine particle size silica are blended for 30 minutes. After the blending is completed 389 parts by weight of 98% H 0 oxidizer are added to the blend and the blending is resumed for an additional l5 minutes. A gel-like propellant having a specific impulse approximately 335 is produced.

A 18 parts by weight portion of polyethylene particles is blended for 20 minutes with 3 parts by weight of silica (7l-l0 millimicrons and 300-350 sq. meter/- gram). At the end of this time a 79 parts by weight portion of perchloryl fluoride (ClO F) is blended in for an additional 20 minutes. The resultant propellant is a thick gel having a specific impulse of approximately 280 and is relatively stable upon storage at room temperature.

A 125 parts by weight portion of polypropylene balls and 5 parts by weight of finely divided silica (described above) are blended for minutes as previously described. To the blended mixture are added 82.5 parts by weight of H 0 (98%) oxidizer and the mixture is blended for minutes until a uniform propellant formulation is produced. Once again the formulated mixture produced a highly energetic propellant with a specific impulse of about 275.

The embodiment immediately above is substantially repeated except that powdered polypropylene is used as fuel and the components are present in the parts by weight specified: polypropylene, 19 parts by weight; silica, 3 parts by weight; and N 0 78 parts by weight. Once again an energetic propellant having above average specific impulse is obtained.

Other embodiments are:

A 56 parts by weight portion of finely divided Li Si and 4 parts by weight of finely divided SiO (described above) are blended for 30 minutes until a uniform mixture is obtained. The blended mixture of fuel and thickener is blended into 72 parts by weight of concentrated (98%) H 0 for an additional 30 minutes. A highly energetic, thixotropic propellant is obtained.

A I92 parts by weight portion of AhSi" and 20 parts by weight of finely divided carbon black (described earlier) thickener are blended for 30 minutes until a uniform dispersion is obtained. The blended mixture is added to and blended with 257 parts by weight of HIRFNA oxidizer for an additional 30 minutes. A highly energetic thixotropic propellant mixture is obtained.

As indicated by the many illustrative embodiments disclosed throughout this application, numerous modifications and variations can be made in the invention without departing from the inventive concept. Thus the metes and bounds of this invention can best be determined by an examination of the claims which follow.

We claim:

1. Novel thixotropic, monopropellant compositions comprising a mixture of a solid fuel selected from the group consisting of inorganic silicides, silicon carbide and mixtures thereof, a strong liquid oxidizer and a thixotroping agent selected from the group consisting of particulated carbon and silica.

2. The thixotropic monopropellant compositions of claim 1 wherein the components are present in about the following proportions:

15-50 parts by weight of solid fuel 50-95 parts by weight of liquid oxidizer and up to 10 parts by weight of thixotroping agent.

3. The thixotropic monopropellant compositions of claim 2 wherein up to 6 parts by weight of propellant adjuvants are present.

4. The thixotropic monopropellant composition of claim 1 wherein said fuel is lithium silicide.

5. The thixotropic monopropellant composition of claim 1 wherein said fuel is aluminum silicide.

6. A thixotropic monopropellant composition comprising a mixture of:

from about l5-50 parts by weight of aluminum hydride from about 50-95 parts by weight of liquid oxidizer selected from the group consisting of concentrated H 0 N 0 and ClO F, and up to l0 parts by weight of thixotroping agent selected from the group consisting of particulated carbon and silica. 

1. NOVEL THIXOTROPIC, MONOPROPELLANT COMPOSITIONS COMPRISING A MIXTUREOF A SOLID FUEL SELECTED FROM THE GROUP CONSISTING OF INORGANIC SILICIDES, SILAICON CARBIDE AND MIXTURES THEROF, A STRONG LIQUID OXIDIZER AND A THIXOTROPING AGENT SELECTED FROM THE GROUP CONSISTING OF PARTICULATED CARBON AND SILICA.
 2. The thixotropic monopropellant compositions of claim 1 wherein the components are present in about the following proportions: 15-50 parts by weight of solid fuel 50-95 parts by weight of liquid oxidizer and up to 10 parts by weight of thixotroping agent.
 3. The thixotropic monopropellant compositions of claim 2 wherein up to 6 parts by weight of propellant adjuvants are present.
 4. The thixotropic monopropellant composition of claim 1 wherein said fuel is lithium silicide.
 5. The thixotropic monopropellant composition of claim 1 wherein said fuel is aluminum silicide.
 6. A thixotropic monopropellant composition comprising a mixture of: from about 15-50 parts by weight of aluminum hydride from about 50-95 parts by weight of liquid oxidizer selected from the group consisting of concentrated H2O2, N2O4, and ClO3F, and up to 10 parts by weight of thixotroping agent selected from the group consisting of particulated carbon and silica. 